1. Field of Invention
This invention relates to load transfer joints in pavement, to ensure that the crack extends straight down through the middle of the dowel bar and that the load is evenly distributed across the joint.
2. Description of Prior Art
Load transfer across a joint in concrete pavements, is the ability of the joint to transfer a hundred percent of the load across the joint. Factors contributing to load transfer across the joints include aggregate interlock, mechanical load transfer devices, such as a dowel bar and a stabilized sub-base.
Transverse contraction joints are constructed transverse to the centerline. The transverse contraction joints are spaced to control cracking from stresses caused by shrinkage, moisture changes and thermal changes. Dowel bars are placed in across the transverse contraction joint to distribute load over the pavement joint. The dowel bar is a smooth steel rod that provides vertical shear resistance across the joint. Dowel bars are typically placed during construction either in wire baskets assemblies or set in place by an automatic dowel bar inserter. The dowel bar assemblies are frames made out of wire, that support dowels at the proper location. Properly placed dowel bars reduce the stress and deflection on the concrete pavement and the potential for faulting, pumping and corner breaks of the joint. The size of the dowel ranges from 1.25 inches for a pavement slab thickness of up to 10 inches and 1.5 inches diameter for slabs greater than 10 inches thick. The dowels are typically 15 inches to 18 inches long, with an embedment length of 6 times the diameter of the dowel bar. The dowels are placed at mid depth of the pavement slab and are spaced typically at 12 inches apart.
Concrete shrinkage starts shortly after the placement of the concrete pavement. The temperature change in the concrete is the main reason for the shrinkage. Heat of hydration of the cement and temperature change in the concrete pavement typically peaks a short period after the final set of the concrete. Once the concrete temperature has peaked, the core temperature in the pavement will start to decline. This typically happens 12 to 24 hours after the concrete in the pavement is placed. This change in temperature typically induces strain in the pavement and cracks are formed. In order to control the cracking in the pavement, a saw cut is made at regular intervals in the pavement. The saw cut provides a weakened plane in the pavement, which in turn is where the pavement to crack. The saw cut controls the location of the crack in the pavement. It is recommended that the saw cut be approximately ¼ the thickness of the pavement. The saw cut and crack form a controlled joint in the pavement. The saw cut is typically located over the center of the dowel bar. The crack extends from the bottom of the saw cut through the dowel bar to the bottom of the pavement. The crack does not always extend straight down to the bottom of the pavement. The crack extends typically in a diagonal direction down to the bottom of the pavement. This results in a short and long section of the dowel bar providing shear resistance across the joint, which results in higher stresses at the shorter end of the dowel bar. The higher stress in the pavement results in a shortened the life of the pavement.
The location of the saw cut determines the origin of the crack. A saw cut made at the wrong location will result in a crack originating off centered with respect to the position of the embedded dowel bars. The off centered crack will result in a long and short section of dowel bar providing shear resistance across the joint. This results in higher stresses at the shorter end of the dowel bar, which in turn shortens the life of the pavement.
The industry recommendation for the depth of the saw cut is to be one-third to one-quarter the overall thickness of the pavement. The depth recommendation is intended to ensure that the pavement cracks, but does not control the direction of the crack. There is no mechanism to ensure that the pavement crack extends vertically from the bottom of the saw-cut to the bottom of the pavement. A diagonal crack in the pavement joint causes higher stress in the shorter section of the dowel bar, a reduction on the embedment length of the dowel bar in the pavement and reduces the useful life span of the pavement. It is also very difficult to ensure that the depth of the saw cut is satisfactory, since not every saw cut joint is inspected.
Aggregate interlock is defined as “the interlocking action between aggregate particles at the face of the joint.” Aggregate interlock is not typically sufficient to transfer a hundred percent of the load across the joint. Thereby a combination of aggregate interlock and dowel bars are typically used to ensure that a hundred percent of the load is transferred across the joint. In order to maximize the load transfer abilities across the joint, a shallower saw cut is needed. This increases the overall surface area of the crack, thereby maximizing the surface area to allow for aggregate interlock. A deeper saw cut will result in a reduction in the overall area for aggregate interlock to occur.
With the current methods of construction, a saw cut less than ⅓ the overall depth of pavement, may not be sufficient to create the weakened plane in the pavement. This results in cracking elsewhere in the pavement, and not at the saw cut. This renders the dowel bars ineffective.
A summary of disadvantages of prior art are:                (a) The saw cut over the dowel bar does not control the direction of the crack.        (b) A diagonal crack results in unequal embedded sections of dowel bars, thus creating higher stress on the shorter section of the dowel bar.        (c) A saw cut made at the wrong location will result in a crack originating off centered with respect to the position of the embedded dowel bars and will result in the crack initiating off centered with respect to the dowel bar. The precise location of the saw cut is highly critical.        (d) A shorter dowel bar section does not meet the embedded length requirements.        (e) Deep saw cut reduces the overall surface area of the crack and thereby reduces the effectiveness of aggregate interlock to transfer the load across the joint.        (f) The expected lifespan of the pavement is typically compromised because of one or more of the above mentioned factors.        